This invention relates in general to communication systems, and more particularly, to the conditioning of a modulated signal, such as for amplification purposes.
Contemporary communication systems often employ modulated signals to effect communications. For example, in a typical digital radio communication device, the transmitted source information is represented by a digital information stream. This digital information stream is modulated and amplified for transmission over a communication channel. Many complex digital modulation schemes have been developed to efficiently convey information across a communication channel. Depending on the digital modulation scheme used, a resultant transmitted signal may have a signal envelope with substantial variation or dynamic range. The dynamic range of the transmitted signal envelope affects the design and selection of a power amplifier used to amplify the communication signals before transmission.
Typically, the power amplifier must accommodate the variations in the signal envelope without distorting the transmitted signal. Distortion of the transmitted signal can cause undesirable effects, such as spectral spreading of the signal energy into adjacent communication channels and degradation to the receiver sensitivity. To avoid signal distortion, the power amplifier is designed to linearly amplify the transmitted signal over its complete dynamic range.
The operating characteristics of conventional power amplifiers dictate that the amplifier efficiency increases monotonically with the value of the signal envelope. A constant signal envelope modulation, such as frequency modulation (FM), allows a power amplifier to be designed to operate continuously at peak efficiency. However, when the value of the transmitted signal envelope varies with time, the overall amplifier efficiency will be significantly less than peak efficiency. In a battery powered communication device, this reduction in amplifier efficiency results in reduced battery life. Amplifiers that accommodate large signal dynamic ranges are also relatively expensive to develop and produce, when compared to amplifiers for constant signal envelope signals.
Known power amplification techniques such as Doherty, Supply Modulation, and LINC (linear amplification with nonlinear components), are designed to amplify signals having a varying signal envelope without distorting the signal, while simultaneously providing improved power efficiency. However, with these amplification techniques, it is expensive to accommodate signals with a large dynamic range while maintaining good performance. Consequently, use of a particular modulation scheme with a varying signal envelope may preclude the selection of certain amplifier designs because of cost and performance issues.
The reduction of power consumption has become an increasingly important aspect of the design of a radio communication device. The provision of an efficient power amplification for transmitted signals is a critical component in reducing power consumption. However, some modulation schemes used to maximize spectral efficiency may have 60 decibels or more amplitude dynamic range, which limits the ability to use efficient amplification techniques. It is desirable to facilitate the efficient amplification of modulated signals while avoiding problems associated with prior art devices.